The present invention relates to a method for detecting and evaluating the optical properties of a specimen and a device for carrying out the method. The method uses a light source that has alternating light and dark phases, a detector that picks up light from the light source by way of the specimen being examined, and a circuit that integrates and digitalizes the signals received from the detector, with the signals emitted from the detector being integrated both during at least part of the light phase and during at least part of the dark phase, with the integral obtained during the dark phase being subtracted from that obtained during the light phase in order to determine the results, with a constant voltage of opposite polarity being applied to, and integrated down by, the integrator in order to convert the integral into a digital signal, and with a counting process triggered at the commencement of down-integration and stopped when a comparator value is attained.
The employment of photometric analysis, especially in the field of medicine and often utilizing test strips, is constantly increasing. Such procedures exploit chemical reactions that occur between the components of body fluids like blood or urine and reagents impregnated into certain reaction areas on the strip, which change color as a result. The changes in color can be quantitatively evaluated by appropriate procedures or devices. A reflected-light photometer for example can determine the reflectivity of such an area at one or more wavelengths of light subsequent to a reaction. In addition to this particular property of the specimen, a reference property and, if necessary, one or more additional specimen properties may have to be evaluated, depending on the given conditions.
Of course, types of analysis that do not employ strips also continue to be used in clinical chemistry. A change of color indicating a reaction that occurs within a more substantial volume of liquid may for instance need to be evaluated. Such methods employ transmitted-light photometers, and the invention may also be employed with this type of photometer.
The invention is preferably employed, however, with a reflected-light photometer, which is the embodiment that will be specified. This type of photometer includes an Ulbricht globe that uniformly distributes the light from a light source so that the test strip will be illuminated with diffused light passing through an aperture. The globe also contains two photodetectors, preferably photodiodes, one, the specimen-property detector, which detects light derived from the reaction area and the other, a reference-property detector, which detects light derived from an unaffected area of the inside surface of the globe. Since the detectors convert the photometric properties into very weak currents, of the order of 1 nA, the results must subsequently be amplified before being evaluated.
It is in clinical chemistry in particular that extraordinarily high demands are made on the detection and evaluation of photometric signals. Oscillations of 1 pA must be reliably detected to measure such weak currents with adequate resolution. There are, however, a number of sources of interference with this objective. They include problems like amplifier drift, superposed DC voltages (especially offset voltages from the components), relatively high-frequency noise voltages, low-frequency AC voltages, especially hum voltages spread with power-supply frequency, amplifier-input frequencies, and leakage or residual currents of various types, that are familiar in the electronics of detecting very weak signals.
Added to this are the sources of interference, especially from outside or extraneous light, that are especially typical in optical detection systems. Comparatively constant DC-voltage components are observed next to interference that is typical of extraneous light sources powered by the main supply equipment and operating at a conventional power-supply frequency of 50 or 60 Hz.
A number of methods of suppressing these sources of interference have been proposed. It is especially common to activate and deactivate the light sources at timed intervals, either by turning them on and off or by interposing and removing a screen. The signals obtained from the detector are then evaluated in the same way during both part of the light phase and part of the dark phase. Interference that contributes additively to the signal from the specimen-property detector and that changes slowly in relation to the rate at which the light source is activated and deactivated can then be substantially eliminated by subtracting the results obtained during the dark phase from those obtained during the light phase.
Methods that integrate the signal coming from the detector over a particular length of time are especially appropriate for suppressing high-frequency interference. Methods of this type will extensively suppress interference with frequencies that are much higher than the reciprocal of the period of integration.
A method and device of this type that combine both measures are known from U.S. Pat. No. 4,201,472. A detector is illuminated by a timed light source and the resulting signals are integrated during both part of the light phase and part of the dark phase. The integrals are evaluated immediately after each integration. This is done by applying a constant comparison voltage to the integrator and using it, analogously to an analog-to-digital converter that functions on the principle of double integration, for down-integration while simultaneously triggering a counting process. A comparator compares the output voltage of the integrator with a comparison voltage, usually a zero potential, and the counting is interrupted as soon as the output of the integrator reaches the zero potential. In the known method, the signals coming from the detector are evaluated during both the light phase and the dark phase, digitalized, and one result subtracted from the other to obtain a corrected value.
This known method, however, is still not completely satisfactory even though it eliminates both high-frequency interference as well as some of the low-frequency interference. Circuitry that will provide satisfactory precision is relatively expensive because the light source must be activated and deactivated fairly rapidly to suppress relatively high-frequency oscillations in the signal. This will leave a comparatively short time for integration during both the light and the dark phases, although the resolution of an analog-to-digital converter that operates according to the double-integration principle is to a large extent determined by maximum available down-integration time at a given counter rate and switching speed. If down-integration time is very short, the only solution is to employ correspondingly rapid components, which will make the device too expensive.